1. Field of the Invention
The present invention relates to a method of creating a scored metal panel such as a metal container lid through sequential scoring operations which provide improved performance properties for the container lid and it also relates to associated apparatus for effecting such scoring and the scored lid.
2. Description of the Prior Art
It has been known for many years that metal panels may be provided with thinned zones in order to facilitate subsequent fracture of such zones. A particular known application is in connection with containers which have embodied what has been called "an easy open end." For example, it has been known to provide thinned zones which define regions that may be totally or partially removed in order to permit access to the contents of beverage and food cans. See, generally, U.S. Pat. Nos. 3,650,006; 3,701,330; 3,728,980 and 3,946,683.
It has been known to provide a single indenter tool having a trapezoidal profile which has a generally flat base with a pair of upwardly diverging straight sidewalls in order to thin the metal panel within a predetermined region which is adapted to be totally or partially removed in opening a container. It has also been known to secure various types of lever tabs to such scored zones so as to provide a mechanical advantage to the user in effecting fracture and opening of such containers. Typically, an organic coating is provided on the interior of such a container lid so as to resist undesired corrosive attack of the container lid material by the contents of the container.
As is apparent, in designing such container walls it is desired to achieve an optimum balance between sufficient depth of scoreline to facilitate ease of opening by the consumer, while having adequate residual metal in the scoreline so as to preserve the required strength of the container lid during double seaming to the container, processing, storing, shipping, handling and ultimate use by the consumer. Such need is acute where the contents consist of a pressurized beverage such as a soft drink or beer and in food cans which may be vacuum packed or not.
In conventional scoring processes the scoring action produces an undesirable localized shear strain and residual strain in and adjacent to the scoreline. These regions are prone to undesired cracking and provide a preferential corrosion path. Even if an initial crack does not occur during sealing, a typical container is subjected to repeated cycles of applied force during processing, shipment, handling and storage. This repeated loading can result in metal fatigue that creates undesired cracking and leakage in the zone of increased strain created by the scoring operation.
The problem is acute with respect to alloys which tend to develop zones of high localized shear straining in this type of application, such as the 5 xxx aluminum alloys, for example, such as 5042 and 5182 and other high strength alloys which otherwise would be highly advantageous in such applications. Such high strength alloys, however, have limited formability as a result of their high strength and can provide highly localized shear strain distribution as a result of present scoring processes.
Also, a further problem resulting from shear strain is the fact that not only may partial destruction of the underlying organic protective coating result in corrosion, but also in the highly strained bands, the metal is more vulnerable to corrosion.
In addition to the hereinbefore described trapezoidal type profile for a scoring tool, a number of other configurations have been suggested. For example, U.S. Pat. Nos. 3,650,006 and 3,701,330 seek to provide improved abuse and fatigue resistance by eliminating the dead zone of compressed work-hardened metal underlying conventional scorelines. The disclosure suggests the use of a score tool having a predetermined included angle between lateral faces of the indenter tool which preferably is said to be about 80 to 100 degrees.
U.S. Pat. No. 3,728,980 discloses a scoring tool having a five-sided profile with a narrow tool base.
U.S. Pat. No. 3,954,075 discloses a scoring tool that provides a pair of anti-fracture scorelines laterally spaced from and of lesser depth than the principal scoreline.
U.S. Pat. No. 4,487,539 discloses effecting scoring from both sides of the metal material.
U.S. Pat. Nos. 3,898,944 and 4,012,935 disclose tools having a narrow scoring portion which first engages the metal followed by a broader based indenter tool surface.
U.S. Pat. No. 4,348,464 discloses a combination of scoring tools which have a stepped scoring tool cooperating with an underlying anvil having a pair of spaced scoring rims.
U.S. Pat. No. 3,359,773 discloses a scoring tool having a central depending indentor and a pair of laterally spaced bending elements. The underlying anvil has an upwardly projecting rib which is adapted to engage the metal in generally aligned relationship with the central scoring portion.
U.S. Pat. No. 3,946,683 relates to a table weakened wall container which wall has a plurality of weakened portions.
In spite of the foregoing known systems and products, there remains a real and substantial need for a method and associated apparatus for scoring a metal panel such as a metal container lid which will provide increased resistance to failure while maintaining improved opening characteristics.